Two abyss holes still incomplete, but only 1 monster left on map

Bug Report ID: #851243071
T1 waystone, Marrow map. I've cleared the map other than abysses. There are two final pits still open, and not close to each other. I figured that there was an invisible monster for each, or something I hadn't triggered to come off the edge, like a spider. However, each abyss that doesn't complete should have a monster associated with it, and after killing the boss, it says only one monster remains.